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Alex Pogossov Alex Pogossov is offline
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Default Triode or pentode with local NFB?

The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?

Regards,
Alex


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John L Stewart John L Stewart is offline
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Location: Toronto
Posts: 301
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Quote:
Originally Posted by Alex Pogossov View Post
The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?

Regards,
Alex
I bench tested a cct such as you have described a few years ago. Was a 6V6 driven by a 6SJ7. NFB from the 6V6 plate thru a FB R to form part of the 6SJ7 plate load. The FB factor is the ratio of the FB R to the plate load R, following grid R & Rp all in parallel.

Why use a pentode driver? A triode as the driver will distort more of its OP signal as it drives into the NFB signal, so in my opinion not a good choice. The pentode is a current source, so noproblem. But the resulting cct is sensitive to power supply hum & noise because the plate is now a low Z point. All of the PS hum & noise will appear in the OPT primary.

But over many years & several SEP amplifiers I've found the best bet is simply a FB pair consisting of the OP pentode thru a reasonable OPT, all driven by a gain stage of triode or pentode depending on how complicatedone wants to go. Then10 to 20 db NFB around the entire cct. And no longer sensitive to PS disturbances,

As usual there are several roads to a reasonable conclusion. For now I prefer this one. Others may freely disagree!

Cheers to all, John
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patrick-turner patrick-turner is offline
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Default Triode or pentode with local NFB?

On Wednesday, 7 November 2012 10:33:50 UTC+11, Alex Pogossov wrote:
The recent thread about the SET 300B based amplifier got me to question: Why triode? 1) Obviously, a triode 300B boasts low distortion say 2% at 6....8W output and reasonable damping WITHOUT GNFB, but the penalties a low plate efficiency, complexity of filament supply, cost, possible self-destruction if shorted load, full volume and fixed bias.


The use of 300B satisfies those who insist only real triodes in pure class A should handle their ear input signal. All later developments in audio amps after 1930 were tainted by economic considerations and silly arguments about efficiency. Some folks still buy what other laugh at.


2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in parallel) design needs GNFB to achieve the same benchmarks, but more efficient, cheap and will not blow the tubes. A penalty here is probably the GNFB loop which encompasses the OPT and would require shelving or other elaborate compensation, which only Mr Turner can properly master to achieve stability over the wide range of the load impedance.

I'm not the only expert on tailoring open loop gain with shelves while keeping BW wide as possible into pure R loads and while allowing any pure C load to be used without more than 3dB peaking in response above 25kHz.

People have been using GNFB around pentode amps since they were invented. But the pentode gives a spray of multiple H in its THD, and RDH4 compares a 6F6 with 2A3 if you wish to know the typical differences. GNFB around OPT and pentodes reduces all H products, and about 20dB is needed so that a typical SEP giving 12% THD at -1dB is reduced to about 1.4%. Many people would rather have 4W from a 2A3 than the same 4W from a 6V6, and not have to use so much FB, if any.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with local feedback. In effect the goal here is to reduce the gain (from G1 to plate) of the stage to about the typical mu of a rival triode. In other words, replace the internal electric field local NFB in a triode by an external local feedback around a pentode. One way is to use cathode feedback with the cathode winding having about 1/4 of number of turns of the plate winding. A drawback here is a special transformer to be made. Another simpler method is to arrange a resistive local feedback. Throw say R1=470K from the output tube plate to grid, through a DC blocking cap of course. Feed the input signal to the grid via a series R2=150K resistor. Thus the output stage will resemble an inverting op-amp, the gain of which is defined by the ratio of R2 / R1. This is because an inherent gain of a pentode from grid to plate is high. Say for two EL34s in parallel it can be up to 50. (More practical to throw R1 from the output tube plate to the driver tube plate without a DC blocking cap.) Thus the local feedback can reach 15...20dB. As a result: a) Drive voltage will be comparable to 300B; b) Distortion will be same if not lower than with 300B; c) The local NFB will be unconditionally stable, since it does not include OPT; d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1) + 1). With a paig of hi-gm tubes as EL34s it will be even lower than with 300B; e) Will yield higher efficiency with the same DC input power; f) No real need for resonance damping circuits across OPT primary; g) No GNFB will be needed; h) Not prone to self destruction with shorted load. A drawback though is a low input impedance (about R2). However, with a cathode follower in the driver stage it will not be an issue. I am wondering if such "op-amped" pentode stages are common. What am I missing? Regards, Alex

I have made 30W+ SEP amps with both 4 x 6AC7 and 1 x 13E1 and they are fully detailed at http://www.turneraudio.com.au/se35cfb-monobloc.htm and http://www.turneraudio.com.au/monobl...12version.html

These amps employ local NFB in OP stages based on the Acoustical Connection where a substantial % of OPT primary turns are used in the cathode circuit.. I also like to sometimes use some screen signal, ie, the tubes have operating voltages conforming to Ultralinear, so that odd no H are reduced to be similar to triode and then the reasonably high gain of UL still allows the CFB windings to then reduce OP tube gain to about 3.3, so that to make 200Vrms Va to Vk, some 60Vrms must be applied to the OP grids. But where there is a high amount of CFB, say 33% as I did with 13E1, and if g2 is taken to a fixed Vdc, then you have UL action because of the signal between screen and cathode.

Then I find the OP stage works like triode but with lower Ra and THD and the driver can be made to be linear as is convenient to do if you have a choke dc feed to driver anode, and you still get 2H cancelling. Then GNFB can be quite low, but total GNFB + local voltage NFB is about 20dB, but the result in THD is better than if the tubes were set up with cathode AND screen both bypassed to 0V and there was only 20dB GNFB. I don't have to worry about my ideas being stolen or copied, I'm a just a fukken nobody who promotes fancy ideas that are damn difficult to implemement, and which all cause apolexy in accounting and tranny winding departments of amp making companies, even in China. Some ppl have tried making made amps using my ideas, but most give up. And those that get them working without FB then struggle with stability caused by them having a different OPT to what I've used, or they change other things and they just NEVER understand 1,001 basic issues such as motorboating, or that their amp is a band pass filter around which one uses NFB. There is much that cannot be fully explained, and blokes have got to realise that if they **** about with amps they end up with oscillators if they just don't get it fully. No wonder SET triode amps are popular with the largely ignorant DIY Brigade of the Unskilled.
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Alex Pogossov Alex Pogossov is offline
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Default Triode or pentode with local NFB?


"patrick-turner" wrote in message
...
On Wednesday, 7 November 2012 10:33:50 UTC+11, Alex Pogossov wrote:
The recent thread about the SET 300B based amplifier got me to question:
Why triode? 1) Obviously, a triode 300B boasts low distortion say 2% at
6...8W output and reasonable damping WITHOUT GNFB, but the penalties a
low plate efficiency, complexity of filament supply, cost, possible
self-destruction if shorted load, full volume and fixed bias.


The use of 300B satisfies those who insist only real triodes in pure class A
should handle their ear input signal. All later developments in audio amps
after 1930 were tainted by economic considerations and silly arguments about
efficiency. Some folks still buy what other laugh at.


2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

I'm not the only expert on tailoring open loop gain with shelves while
keeping BW wide as possible into pure R loads and while allowing any pure C
load to be used without more than 3dB peaking in response above 25kHz.

People have been using GNFB around pentode amps since they were invented.
But the pentode gives a spray of multiple H in its THD, and RDH4 compares a
6F6 with 2A3 if you wish to know the typical differences. GNFB around OPT
and pentodes reduces all H products, and about 20dB is needed so that a
typical SEP giving 12% THD at -1dB is reduced to about 1.4%. Many people
would rather have 4W from a 2A3 than the same 4W from a 6V6, and not have to
use so much FB, if any.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode. One way is to use cathode feedback
with the cathode winding having about 1/4 of number of turns of the plate
winding. A drawback here is a special transformer to be made. Another
simpler method is to arrange a resistive local feedback. Throw say R1=470K
from the output tube plate to grid, through a DC blocking cap of course.
Feed the input signal to the grid via a series R2=150K resistor. Thus the
output stage will resemble an inverting op-amp, the gain of which is defined
by the ratio of R2 / R1. This is because an inherent gain of a pentode from
grid to plate is high. Say for two EL34s in parallel it can be up to 50.
(More practical to throw R1 from the output tube plate to the driver tube
plate without a DC blocking cap.) Thus the local feedback can reach
15...20dB. As a result: a) Drive voltage will be comparable to 300B; b)
Distortion will be same if not lower than with 300B; c) The local NFB will
be unconditionally stable, since it does not include OPT; d) Output
resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1) + 1).
With a paig of hi-gm tubes as EL34s it will be even lower than with 300B; e)
Will yield higher efficiency with the same DC input power; f) No real need
for resonance damping circuits across OPT primary; g) No GNFB will be
needed; h) Not prone to self destruction with shorted load. A drawback
though is a low input impedance (about R2). However, with a cathode follower
in the driver stage it will not be an issue. I am wondering if such
"op-amped" pentode stages are common. What am I missing? Regards, Alex

I have made 30W+ SEP amps with both 4 x 6AC7 and 1 x 13E1 and they are fully
detailed at http://www.turneraudio.com.au/se35cfb-monobloc.htm and
http://www.turneraudio.com.au/monobl...12version.html

These amps employ local NFB in OP stages based on the Acoustical Connection
where a substantial % of OPT primary turns are used in the cathode circuit.
I also like to sometimes use some screen signal, ie, the tubes have
operating voltages conforming to Ultralinear, so that odd no H are reduced
to be similar to triode and then the reasonably high gain of UL still allows
the CFB windings to then reduce OP tube gain to about 3.3, so that to make
200Vrms Va to Vk, some 60Vrms must be applied to the OP grids. But where
there is a high amount of CFB, say 33% as I did with 13E1, and if g2 is
taken to a fixed Vdc, then you have UL action because of the signal between
screen and cathode.

Then I find the OP stage works like triode but with lower Ra and THD and the
driver can be made to be linear as is convenient to do if you have a choke
dc feed to driver anode, and you still get 2H cancelling. Then GNFB can be
quite low, but total GNFB + local voltage NFB is about 20dB, but the result
in THD is better than if the tubes were set up with cathode AND screen both
bypassed to 0V and there was only 20dB GNFB. I don't have to worry about my
ideas being stolen or copied, I'm a just a fukken nobody who promotes fancy
ideas that are damn difficult to implemement, and which all cause apolexy in
accounting and tranny winding departments of amp making companies, even in
China. Some ppl have tried making made amps using my ideas, but most give
up. And those that get them working without FB then struggle with stability
caused by them having a different OPT to what I've used, or they change
other things and they just NEVER understand 1,001 basic issues such as
motorboating, or that their amp is a band pass filter around which one uses
NFB. There is much that cannot be fully explained, and blokes have got to
realise that if they **** about with amps they end up with oscillators if
they just don't get it fully. No wonder SET triode amps are popular with the
largely ignorant DIY Brigade of the Unskilled.

Alex:
I was aware of your cathode feedback design. It requires a specially made
transformer. What I am wondering, do people often use output tubes as
"inverting op-amp" with resistive feedback from plate to grid to make a
rather deep and always stable local NFB around the output stage?


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John L Stewart John L Stewart is offline
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Location: Toronto
Posts: 301
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Quote:
Originally Posted by Alex Pogossov View Post
The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?

Regards,
Alex

This example cct lifted from Audio Anthology shows a 6V6 driven by a 6SH7 pentode connected with both plate to plate & plate to driver cathode NFB.

For the plate to plate NFB connexion the feedback factor would be the 0.56 M FB R driving into the parallel combination of the 6SH7 plate Load R, the following 6V6 grid R & the plate R of the 6SH7. If we take the 6SH7 plate R to be One M that works out to a NFB factor of 0.28 ( 0.15 / 0.56 ).

If the 6V6 is running at 250 volts & the recommended plate load of 5K its gain is about 20. With the plate to plate NFB connexion the gain becomes 3.57, so NFB is about 15 db by this route.

That is the numbers I see. With the other NFB route connected then things get different.

Cheers, John
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John Byrns John Byrns is offline
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Default Triode or pentode with local NFB?

In article ,
"Alex Pogossov" wrote:

The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?


RCA published a paper describing the 6L6 beam power tube and its development in
the Proceedings of the IRE and also the RCA Review back in the mid 1930s.
Besides describing the development of the 6L6, they showed the op amp like
connection you are speaking of and what the characteristic curves of the 6L6
look like when this connection is used, basically triode like IIRC.

I don't think the connection was very common although I believe I have seen a
couple of amps that used it. Much more common is a variant on the connection
where the resistor from the output tube plate is feedback to the driver tube
cathode, I have seen this connection quite a bit over the years.

The problem with either of these schemes is that it destroys the pentodes
inherently good power supply rejection. That means that we either need to
provide greater power supply filtering to reduce the noise on the power supply
line, or go to parallel feed to improve the power supply noise rejection, either
solution adds to the cost.

If I were doing this and was free to spec the output transformer I wanted I
would go with the cathode feedback scheme in the output stage. Taking the
feedback from the plate doesn't improve the low frequency stability issues, it
only helps with high frequency stability, so what I would do if I couldn't spec
the output transformer I wanted, would be to take the negative feedback from the
secondary at low frequencies and from the plate/primary at high frequencies.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
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Alex Pogossov Alex Pogossov is offline
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Default Triode or pentode with local NFB?


"John Byrns" wrote in message
...
RCA published a paper describing the 6L6 beam power tube and its
development in
the Proceedings of the IRE and also the RCA Review back in the mid 1930s.
Besides describing the development of the 6L6, they showed the op amp like
connection you are speaking of and what the characteristic curves of the
6L6
look like when this connection is used, basically triode like IIRC.

I don't think the connection was very common although I believe I have
seen a
couple of amps that used it. Much more common is a variant on the
connection
where the resistor from the output tube plate is feedback to the driver
tube
cathode, I have seen this connection quite a bit over the years.


Interesting to calculate will such feedback be deeper tham the op-amp
connection? Possibly, if Rfb is relatively small (and here you start loosing
AF power) and driver plate load resistor is large. As J.L.Stewart mentioned,
a pentode in the driver will be an advantage.


The problem with either of these schemes is that it destroys the pentodes
inherently good power supply rejection. That means that we either need to
provide greater power supply filtering to reduce the noise on the power
supply
line, or go to parallel feed to improve the power supply noise rejection,
either
solution adds to the cost.


Yes, but all 300B / 2A3 lovers are bound to face the same issue.


If I were doing this and was free to spec the output transformer I wanted
I
would go with the cathode feedback scheme in the output stage. Taking the
feedback from the plate doesn't improve the low frequency stability
issues, it
only helps with high frequency stability, so what I would do if I couldn't
spec
the output transformer I wanted, would be to take the negative feedback
from the
secondary at low frequencies and from the plate/primary at high
frequencies.


I did the two branch feedback exactly in the same manner. One solution is
just to throw a small capacitor (10pF) from the plate to grid. It will work
as a compensation cap in the op-amp, creating a dominant HF pole. In other
words it makes 6V6 work as integrator at HF.

Later I realised that far better is to throw a larger cap from 6V6 plate to
unbypassed cathode of the previous driver stage. (To the same cathode a
second resistive (regular) NFB is fed from the OPT secondary.)

Regards,
Alex


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patrick-turner patrick-turner is offline
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Default Triode or pentode with local NFB?

On Wednesday, 7 November 2012 16:32:59 UTC+11, Alex Pogossov wrote:

Alex: I was aware of your cathode feedback design. It requires a specially made transformer. What I am wondering, do people often use output tubes as "inverting op-amp" with resistive feedback from plate to grid to make a rather deep and always stable local NFB around the output stage?

Using shunt FB with R from anode to cathode then R to some driver source has rarely ever been done in commercially made amplifiers even where output tube gain is high as is the case for normal pure pentode or tetrode output stages.
The reason is that the technique usually requires the driver tubes ahead of the OP stage to have low Ra and to make say 3 times Vg1 applied to OP grid and to cope with R1 which might be say 47k, with R2 to anode being a much higher R and cap coupled to anode. Damn, too many extra R and C and using more expensive drive tubes, so bean counters vomited on the idea. There is an alternative, see pagess 333 and 334 of shunt FB techniques where the high anode Ra of a driver pentode works like R1 of the shunt FB network. ß can then become usefully high as about 0.5, and pentode gain is still high enough. But this idea never took hold either, and the simple cheaper solution of GNFB could never be proven to sound inferior to all manner of other ideas tried by egotists wanting to be original. I tried the idea, see by circuits page for the stuff about Balanced Shunt FB, http://www.turneraudio.com.au/miscel...chematics2.htm


All good tube audio power amps require a "specially made OPT". The non specially made OPTs are all inferior. Quad's OPT for Quad-II was specially made, and no more difficult to make than any other specially made OPT - made by ppl able to do horrid stuff while everyone else is bone lazy.

But, even Quad-II OPTs have no advertised ow loss way to match the amp to 4 ohms, which is needed in 2012 because modern speakers are not often 16 or 9 ohms, but often 5 ohms. So with 5 ohms connected across the 9 ohm setting, RLa-a becomes 2k2, way too low for good fidelity, and then KT66 gain is low, making the 10% CFB fairly ineffective. But its possible to remove Quad-II OPTs from their case, and to adjust sec wires and install an extra terminal to allow waste free sec windings for 4k0 : 4 ohms, so that RLa-a with 5 ohms becomes 5k0, and then the losses reduce and although still high, fain and FB is more effective and overall function much better. Using 8 ohms connected to a 4 ohm OPT setting gives RLa-a = 8k0, and then the Quads function far better, although PO is limited, but its mostly pure class A.

My 300 Watt amps have large OPTs with 6 sec sections 5 primary sections. The core is 110mm stack of 50T E&I GOSS. Np = 1,060 turns in 10 layers so each P section = 2 layers. One complete 2 layer section of P is devoted to CFB = 20% with CT which is far more effective than the 10% of Quad. Now there is NO extra special effort to provide the 20% CFB winding. The OPT can be used to give a wide range of primary use, plain UL, UL with CFB, CFB only, whatever, its doable, and the secs have each section of one layer being 48t + 24t which allow a considerable number of waste free low loss sec connections which maintain the possible bandwidth at 250Watts = 20Hz to 270kHz. The 300W OPT isn't hard to wind using such large dia wire, but there are many connections to make, all of which make CEOs and accountants have heart attacks.

I was once offered awful OPTs by someone for a price way too high, and I said, "Have you ever thought of re-naming your company to 'Mediocre Transformers P/L'? They were not amused, and I just wound all my own after that. They went broke 4 years later, an all too common occurence because nothing is specially done for anyone. For them, special = ruiniation.

Patrick Turner.




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patrick-turner patrick-turner is offline
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Default Triode or pentode with local NFB?

On Friday, 9 November 2012 19:07:56 UTC+11, Alex Pogossov wrote:
"John Byrns" wrote in message ... RCA published a paper describing the 6L6 beam power tube and its development in the Proceedings of the IRE and also the RCA Review back in the mid 1930s. Besides describing the development of the 6L6, they showed the op amp like connection you are speaking of and what the characteristic curves of the 6L6 look like when this connection is used, basically triode like IIRC. I don't think the connection was very common although I believe I have seen a couple of amps that used it. Much more common is a variant on the connection where the resistor from the output tube plate is feedback to the driver tube cathode, I have seen this connection quite a bit over the years.. Interesting to calculate will such feedback be deeper tham the op-amp connection? Possibly, if Rfb is relatively small (and here you start loosing AF power) and driver plate load resistor is large. As J.L.Stewart mentioned, a pentode in the driver will be an advantage. The problem with either of these schemes is that it destroys the pentodes inherently good power supply rejection. That means that we either need to provide greater power supply filtering to reduce the noise on the power supply line, or go to parallel feed to improve the power supply noise rejection, either solution adds to the cost. Yes, but all 300B / 2A3 lovers are bound to face the same issue. If I were doing this and was free to spec the output transformer I wanted I would go with the cathode feedback scheme in the output stage. Taking the feedback from the plate doesn't improve the low frequency stability issues, it only helps with high frequency stability, so what I would do if I couldn't spec the output transformer I wanted, would be to take the negative feedback from the secondary at low frequencies and from the plate/primary at high frequencies. I did the two branch feedback exactly in the same manner. One solution is just to throw a small capacitor (10pF) from the plate to grid. It will work as a compensation cap in the op-amp, creating a dominant HF pole. In other words it makes 6V6 work as integrator at HF. Later I realised that far better is to throw a larger cap from 6V6 plate to unbypassed cathode of the previous driver stage. (To the same cathode a second resistive (regular) NFB is fed from the OPT secondary.) Regards, Alex


I've tried caps between OP anodes and g1 or to driver cathodes, but usually it just causes more open loop phase shift of 90 degrees at a lower F than do the existing amounts of C shunting Ra in the input-driver amp stages. So with the ultimate OLG phase shift of 180d occuring at a lower F the amp becomes harder to stabilise with other networks with a pure C load. The C you put there adds a high Miller effect where it was smaller to begin with. But perhaps its possible to use an R&C zobel to shelve the HF that way to avoid the 180d shift at such a low F. The R&C zobel is best as a passive network across V1 anode to 0V, ie, it shunts Ra, and this works better IMHO to reduce ringing on C loads and keep away from FB C having to load down any driver stages.

I'll do it your way after I have seen your many schematics which show/prove your method to be superior. When I test many amps it is revealing to set them up with a 0.22uF cap load and with 5kHz square wave at the -12dB level. One will often see some ringing at Vo which is to be expected. But if you examine the anodes of OP tubes, the ringing is often a lot worse than at Vo.. Just what is done to reduce the ringing is debateable, but I recall having tried every possible way without reducing the widest full power BW with pure R load but while making the amp unconditionally stable with an possible load or none at all. Whay appears in schematics at my site is a distilation of ideas that worked, and priciples that can be applied to nearly all amps without wasting too much of my precious time.

Patrick Turner.
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Quote:
Originally Posted by John Byrns View Post
In article ,
"Alex Pogossov" wrote:

The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?


RCA published a paper describing the 6L6 beam power tube and its development in
the Proceedings of the IRE and also the RCA Review back in the mid 1930s.
Besides describing the development of the 6L6, they showed the op amp like
connection you are speaking of and what the characteristic curves of the 6L6
look like when this connection is used, basically triode like IIRC.

I don't think the connection was very common although I believe I have seen a
couple of amps that used it. Much more common is a variant on the connection
where the resistor from the output tube plate is feedback to the driver tube
cathode, I have seen this connection quite a bit over the years.

The problem with either of these schemes is that it destroys the pentodes
inherently good power supply rejection. That means that we either need to
provide greater power supply filtering to reduce the noise on the power supply
line, or go to parallel feed to improve the power supply noise rejection, either
solution adds to the cost.

If I were doing this and was free to spec the output transformer I wanted I
would go with the cathode feedback scheme in the output stage. Taking the
feedback from the plate doesn't improve the low frequency stability issues, it
only helps with high frequency stability, so what I would do if I couldn't spec
the output transformer I wanted, would be to take the negative feedback from the
secondary at low frequencies and from the plate/primary at high frequencies.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
Here are the curves with Local NFB as you have referenced.
Attached Images
 


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patrick-turner patrick-turner is offline
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Default Triode or pentode with local NFB?

On Saturday, 10 November 2012 01:23:43 UTC+11, John L Stewart wrote:
John Byrns;963727 Wrote:


snip

Here are the curves with Local NFB as you have referenced. +-------------------------------------------------------------------+ |Filename: 6L6 Local NFB Curves RDH4 p396.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=313| +-------------------------------------------------------------------+ -- John L Stewart

The curves show that at Ia at 70mA, Ea = 350V, Ra' = 2k0 approximately.
Ra without FB is variable, but approx 33k, and gm = 5mA/V approx so µ = 165
Ra' = Ra / ( 1 + [ µ x ß ]) = 33k / ( 1 + [165 x 0.1]) = 1.88k.
Obviously, just how you read the curves gives a variable answer, And Ra gets lower for higher Ia. But setting up a tube and measuring it is the best way to know.

Using a shunt FB network with two resistors requires more voltage drive than the use of series voltage FB with a CFB winding based on keeping the same ß for both applications of FB. In other words, for the same amount of applied FB and Ra', the shunt FB is less sensitive. Plus the shunt FB has low Rin.

But there is one advantage with the shunt FB. The FB is derived with resistors which have no leakage inductance, so that the HF response is very extended and flat without ringing. Say you have an EL34 with FB network R1 = 22k, and R2 = 100k. A DC blocking cap of say 0.47uf could be used between R2 and anode.
In this case ß = 22 / ( 100 + 22 ) = 0.1875. (Its like having 18.75% CFB.) If the EL34 had Va = 200V+ and RL = 4k5, in pentode A = 30, so Vg = 6.7V- so you have 206.7V across 100k and 45.47V- across R1, so Vin ahead of R1 = 45.47 + 6.7 = 52.174V-. The closed loop gain = 200V / 52..174 = 3.8, or about the same as a 300B, and you should find Ra to be slightly lower than a 300B while both tubes would make 9W. Distortion in the EL34 without any FB in pentoad at 9W just under clip would be about 12%, and with FB it should be about 2%, but there are more harmonics present than in the case of 300B, or say a KT88 in triode which works similarly to 300B. Using a UL tap up to say 50% with EL34 in addition to the shunt FB OR the same amount of CFB for the same % of UL, you get slightly higher Ra' and marginally less A' needing maybe 65Vin, and THD remains about the same but H spectra is much more like the triode. The triode 2H without FB is more than with the 18.75% shunt FB around the pentode.
Company bean counters hate paying to make the driver stage produce more voltage at higher current while staying linear to cope with the shunt FB. They prefer to apply global FB which allows OP tubes to be driven easily plus the GNFB reduces all the defects of driver gain and OP tube and OPT - equally, so life is easier with GNFB.

Where you have local FB, AND GNFB, you usually cannot apply much GNFB because despite the local working well, phase shift beyond the ends of the band is rapid for F change, and you end up being constrained so that total of local and global = same = between 12dB and 20dB, so if local FB = say 12dB, then maybe 8dB of global is difficult and still get unconditional stability.
There is not one single free sandwich to be eaten along the journey when travelling with NFB. However, I have repeatedly made amps with CFB and with very wide BW OPTs that many makers never make except perhaps McIntosh and I found it possible to apply up to 35dB GNFB with 12.5% CFB also in OP stage, before the amp became impossible to stabilise.
Patrick Turner.



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